Generally, it is known to provide a subsurface well safety valve for use in a well for shutting off flow of well fluid through the well tubing in which the safety valve is actuated to an open position by hydraulic control fluid from the well surface, is biased to the closed position by a pressurized gas chamber which acts oppositely to the hydraulic control fluid on the operating piston, and uses small rod pistons to provide a low spread between the opening and closing pressures, as shown in U.S. Pat. No. 4,252,197. However, if the piston seal fails, the gas may escape and the valve closing force may become impaired causing a valve failure. Furthermore, in some valves, the tubing pressure or pressure in the housing bore works against the hydraulic control fluid from the surface. This requires increased hydraulic control pressure acting against the tubing or housing pressure which is undesirable, as well as variable, in some applications.
The present invention is directed to various improvements in a low spread piston and cylinder actuated subsurface safety valve having a pressure charged chamber which is constructed to reduce the possible loss of the gas charge in the chamber. A U-shaped gas chamber is exposed to a single seal on the piston which is lubricated on both sides with hydraulic fluid. In addition, the valve is constructed so that the well pressure in the well tubing or housing bore does not affect the opening or closing of the safety valve and thus provides a so-called balanced valve insofar as the effect of well tubing pressure is concerned.